CMake: Moving options below project declaration (#2047)

* move options below project declaration

* expand cmake version dep for c++20 modules

* default VULKAN_HPP_GENERATOR_BUILD to PROJECT_IS_TOP_LEVEL

* fixed format

* moved CMAKE_CXX_STANDARD

* only set cxx standard if top-level

CMakeLists.txt

@@ -24,14 +24,22 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-if ( NOT CMAKE_CXX_STANDARD )
+cmake_minimum_required( VERSION 3.12 )
+project( VulkanHppGenerator LANGUAGES CXX )
+
+if( CMAKE_VERSION VERSION_LESS "3.21" )
+    # https://cmake.org/cmake/help/latest/variable/PROJECT_IS_TOP_LEVEL.html
+    string( COMPARE EQUAL ${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_SOURCE_DIR} PROJECT_IS_TOP_LEVEL )
+endif()
+
+if ( PROJECT_IS_TOP_LEVEL )
 	set( CMAKE_CXX_STANDARD 11 )
 endif()
 
 # all the options for this project
 option( VULKAN_HPP_PRECOMPILE "Precompile vulkan.hpp and vulkan_raii.hpp for sample builds" ON )
 option( VULKAN_HPP_RUN_GENERATOR "Run the HPP generator" OFF )
-option( VULKAN_HPP_GENERATOR_BUILD "Build the HPP generator" ON )
+option( VULKAN_HPP_GENERATOR_BUILD "Build the HPP generator" ${PROJECT_IS_TOP_LEVEL} )
 option( VULKAN_HPP_SAMPLES_BUILD "Build samples" OFF )
 option( VULKAN_HPP_TESTS_BUILD "Build tests" OFF )
 option( VULKAN_HPP_SAMPLES_BUILD_ONLY_DYNAMIC "Build only dynamic. Required in case the Vulkan SDK is not available" OFF )
@@ -41,14 +49,14 @@ option( VULKAN_HPP_ENABLE_CPP20_MODULES "Build Vulkan-Hpp as C++20 module; requi
 option( VULKAN_HPP_ENABLE_STD_MODULE "Build Vulkan-Hpp with import std; requires minimum CMake version 3.30" OFF )
 option( VULKAN_HPP_CPP20_MODULE_DYNAMIC_DISPATCHER "Build C++20 module with dynamic Dispatcher" ON )
 
-if ( VULKAN_HPP_ENABLE_CPP20_MODULES )
-	cmake_minimum_required( VERSION 3.28 )
-else()
-	cmake_minimum_required( VERSION 3.12 )
+if( VULKAN_HPP_ENABLE_CPP20_MODULES )
+	if( VULKAN_HPP_ENABLE_STD_MODULE AND CMAKE_VERSION VERSION_LESS "3.30")
+		message( FATAL_ERROR "Vulkan-Hpp: C++20 modules with import std require CMake 3.30 or later" )
+	elseif ( CMAKE_VERSION VERSION_LESS "3.28" )
+		message( FATAL_ERROR "Vulkan-Hpp: C++20 modules require CMake 3.28 or later" )
+	endif()
 endif()
 
-project( VulkanHppGenerator LANGUAGES CXX )
-
 function( vulkan_hpp__setup_platform )
 	set( options )
 	set( oneValueArgs NAME )